I want to measure the OD of 5 anoxic bacteria culture which have different substrate. I do this for obtaining the growth curve. I mesured the OD but I had some wrong results so I freeze the first 3 samples and I measured by spectophotomer again and the values were what I was expecting. I did the same for another 2 days.

Now I thaw out the samples and when I measure the OD of all and I have absurde results.

Freeze thaw cycles will damage some of your cells leading to release of cellular components, which means that frozen samples will have at best inaccurate readings, at worst readings that don't mean anything at all as the cell components absorb in the same range as bacteria.